BOULDER CREEK ASSEMBLAGE, MOUNT BAKER, WASHINGTON: A RECORD OF THE LATEST CONE BUILDING ERUPTIONS
An 83-meter thick section of the BCA was measured at the type locality on the left bank of Boulder Creek at 490 m elevation, 9 km southeast of the summit crater. At least 12 boulder-sand diamicts, averaging 4 m thick, include andesite clasts to 3 m diameter. Blocks with prismatic jointing indicate that some flows are transitional from block-and-ash flows to granular, water-mobilized lahars. Distal lavas of the BCA are exposed 1 km upstream. Proximal to the cone, lava flows progressively dominate the assemblage. Block-and-ash flows in the type section are likely derived from collapsing fronts of lava flows. Distally, the diamicts and their alluvial runouts debouched from the narrow valley of Boulder Creek into the Baker River valley, forming a fan with a radius of 2.5 kilometers.
Postglacial volcaniclastic assemblages similar in texture to the BCA occur south of Mount Baker, in Sulphur and Pratt Creeks. Fragile, prismatically jointed blocks are conspicuous, but these deposits are not seen to interfinger with lavas.
The BCA is overlain by ash of tephra set SC, (8,800 14C yrs BP)from Schriebers Meadow cinder cone, 8 km south of the Baker summit. Post-BCA Holocene deposits from Mount Baker are juvenile and phreatomagmatic tephras, and lahars originating as flank collapses on the southeast half of the edifice.